The field of this invention is that of tools used for the cleaning of oil and gas pipelines, especially the long extended reach pipelines in offshore areas. As hot production crude is produced from the reservoirs below the ocean floor up to the wellhead equipment at the ocean floor and then thru pipelines along the ocean floor, it is cooled by the relatively cool temperature of the ocean water. In deepwater, the temperature can be as cold as 35 degrees Fahrenheit.
A characteristic common to a majority of the oil produced is a wax or paraffin component to the oil which will deposit on the walls of the pipeline and become a solid at temperatures well above 35 degrees Fahrenheit. Some of the paraffins are a solid at temperatures above 100 degrees Fahrenheit, and so can be deposited or plated on the internal diameters of the pipelines at any expected ambient temperature. The process is similar to blocking the arteries of a human being, with a thicker coating building up with time. Some pipelines have become so plugged that more than 90% of the flow area is blocked with the waxes or paraffins.
Typically, as the pipeline wall becomes layered with paraffin as the temperature of the oil drops below the solidification temperature of the particular paraffins in the produced fluids. The paraffins act as a sort of insulation to the flow in the pipeline, allowing it to maintain a higher temperature for a greater distance. The effect of this is to extend the distance along the pipeline to which the paraffin is plating onto the internal diameter of the pipeline, therefore increasing the pressure drop and flow rate within the pipeline.
A common cure for the paraffin plating out on the internal diameter of the pipeline is to insert a mechanical cleaning device called a “pig” into the flow stream and let the pig remove some of the paraffin. A pig is typically a cylindrical or spherical tool which will brush against the internal diameter of the pipeline in hopes of removing the deposited paraffins. In pipelines with a high incidence of deposited paraffins, a regular pipeline maintenance program with pigs is normally prescribed as a preventative measure to reduce pipeline blockage.
One problem with using pigs is that deposited paraffins are relatively soft and contain large quantities of oil. To some extent, the pigs actually compress the paraffins against the wall and squeeze the oil out, leaving a harder and stronger paraffin remaining.
A second problem is that when the paraffin layer on the internal diameter of the pipe is too thick, sloughing off may occur. If the paraffin starts to separate from the wall and continues, the pig begins to literally plow a block of paraffin ahead of itself. This will continue driving more and more paraffin off the wall of the pipeline until the pressure within the pipeline will no longer be able to move the mass. At that time the pipeline is fully blocked, and the pig cannot be moved by pressure from either end.
When this happens the plug of paraffin must be removed using chemicals. If access to the downstream end of the plug is available, chemicals can be delivered from that end to break down the blockage. Since the paraffin blockage is downstream of the pig, the chemicals cannot be deployed to the plug from the upstream end of the pipeline. If access is only available from the upstream end, there is no way to remove the pig and allow such delivery of chemicals.
Another problem is that if the pig were to be grasped to be pulled back toward the upstream end, its tendency to seal against the wall would cause hydraulic locking and make the pig difficult to remove. At the surface we are accustomed to pulling a vacuum of 14.7 p.s.i. which would result in a 739 lb. force in an eight inch internal diameter pipeline. In a pipeline in 1000 foot of depth of seawater, the ambient pressure is 465 p.s.i. instead of 14.7 p.s.i., yielding a 23,373 lb. force to overcome instead, effectively vacuum locking the pig in place. The vacuum locking in ocean depths can generate massively high forces to overcome.